Network nodes forward data. Network nodes may take form in one or more routers, one or more bridges, one or more switches, one or more servers, or any other suitable communications processing device. The data is commonly formatted as packets and forwarded using forwarding tables. A packet is a formatted unit of data that typically contains control information and payload data. Control information may include: information that identifies sources and destinations, such as addresses, error detection codes like checksums, sequencing information, etc. Control information is typically found in packet headers and trailers. Payload data is typically located between the packet headers and trailers.
Forwarding packets involves various processes that, while simple in concept, can be complex. The processes involved in forwarding packets vary, depending on the type of forwarding method used. Multicast is the preferred method of data forwarding for many networks. One reason for this is that multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering data to multiple receivers. However, some network environments are not well suited to support multicast. Doing so in such environments often involves discovering and maintaining significant amounts of control, or state, information. Setting up and maintaining this control information has a tendency to become complex and costly in terms of computing resources, and can become a major limiting factor in overall network performance. Another issue with multicast is that due to packet delivery mechanisms used, packets are sometimes forwarded to locations to which the forwarding of these packets creates an unwelcome burden on network performance. Overcoming this burden by traditional means involves generation and maintenance of even more control information.
In conventional IP multicast forwarding, the packets of a given multicast “flow” are forwarded along a tree that has been constructed for the specific purpose of carrying that flow. This requires transit nodes to maintain state on a per-flow basis, and requires the transit nodes to participate in multicast-specific tree building protocols. The flow to which a packet belongs is determined by its IP source and destination address fields.
BIER (Bit Index Explicit Replication) is an alternative method of multicast forwarding. It does not require any multicast-specific trees, and hence does not require any multicast-specific tree building protocols. Within a given “BIER domain”, an ingress node encapsulates a multicast data packet in a “BIER header”. The BIER header identifies the packet's egress nodes in that domain. Each possible egress node is represented by a single bit within a bitstring; to send a packet to a particular set of egress nodes, the ingress node sets the bits for each of those egress nodes, and clears the other bits in the bitstring. The set bits are be forwarded and replicated at each network node by flooding an IGP (i.e. the interior gateway protocol) in use in the local network with a unique LSA (i.e. a link state advertisement) so that each routing table/RIB (routing information base) in the local network has the bit assignment. Each packet can then be forwarded along the unicast shortest path tree from the ingress node to the egress nodes. Thus there are no per-flow forwarding entries.